Penguin is a hang on back type. Hagen HOB's work well too because they use block shaped filter media. You can put some biological filter media from the DT' filtration in it when you need it. My favorite piece of equipment for a qt, although expensive, is a uv sterilizer. A diatom filter is cheaper but really messy to deal with.
I also keep pieces of pvc pipe in the qt for fish to hide in.